In-ear (flat head) headphones have continued to be popular due to their lightweight structure. Conventional in-ear (flat head) headphones are, for example, headphone models AirPods and EarPods made by Apple Computer Inc. The in-ear headphones are placed in externally in the ears of a wearer and do not easily transmit sound directly into an ear canal, and therefore sound performance is limited. In addition, conventional technologies do not provide an earplug structure with good sound performance for in-ear (flat head) headphones such as headphone models AirPods and EarPods made by Apple Computer Inc.
Therefore, it is necessary to provide an earplug structure and an earphone device to solve the problems existing in the conventional technologies.